The broad, long term objective of this application is to understand how regulatory proteins of Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) control viral and cellular gene expression during lytic replication. Lytic gene products are likely to have a role in pathogenesis of KSHV, contributing to the ability of KSHV-infected cells and adjacent cells to proliferate, survive and avoid immune responses. Understanding the mechanisms by which lytic gene expression is regulated is therefore relevant to KSHV- related pathogenesis which continues to be a significant cause of morbidity and mortality, particularly in areas with high endemic rates of HIV and KSHV infection. The ORF57 protein of KSHV is expressed early during KSHV replication and is a member of a family of herpesvirus proteins that plays an essential role in herpesvirus replication. ORF57 has unique regulatory properties, post-transcriptionally enhancing expression of many intronless genes. In addition, ORF57 enhances expression of specific cell genes. ORF57 is likely to exert many of its effects by physically binding to mRNA and modulating its stability and nuclear export. Much remains unknown about the mechanismsby which ORF57 affects RNA processing and how its specificity of action is determined. We therefore propose three integrated aims to investigate the role of ORF57 in KSHV biology. In the first aim, we will define the mechanisms by which ORF57 binds mRNA and how ORF57 specificity is determined. In the second aim, we will identify the major cellular proteins that are important for ORF57 function, especially with regard to nuclear export and mRNA processing. In the third aim, we will determine which functions of ORF57 are essential for KSHV replication using molecular genetics. We will also determine the specific effects of ORF57 on gene expression in endothelial cells and B lymphocytes. KSHV is a virus that is linked to the development of cancer, particularly in those people who have weakened immune systems, such as those with HIV infection. Studying how the virus reproduces is important to understand how it affects the cells it infects and converts them to tumor cells. Such studies also have the potential to identify targets for new drugs against the virus that may help prevent or treat infection.